support explicilty calling the next middleware via `next`

[rdmulford]

context

related #30

express provides a nextvariable in middlewares which represents the next middleware function in the middleware stack

see: https://expressjs.com/en/guide/using-middleware.html

this is nice to have because it allows running some code in a middleware after the middleware chain

some useful hints on implementations might be

• https://github.com/expressjs/express/blob/master/lib/middleware/init.js
• https://stackoverflow.com/questions/44238150/why-calling-next-in-express-routes-is-optional

done

• support optional next var in middleware functions